1. Field of the Invention
The present invention relates generally to the replication of original records and, more particularly, to a method of converting a video disc master which contains information in the form of microscopic "holes" in a metal surface into a master suitable for use in producing replicas.
2. Description of the Prior Art
Over the years, there has been a continuing attempt to achieve a low-cost, mass-produced disc which contains video information that can be retrieved with an inexpensive home instrument for playback through a conventional television set. Early attempts at providing video information have generally involved the use of video tape recorders of various sorts as well as photographic techniques. Still other approaches have attempted to utilize thermoplastic recording or the surface alteration of a thin metallic film.
In the copending application of John S. Winslow, Ser. No. 333,560, filed Feb. 20, 1973, and assigned to the assignee of the present invention, a method and apparatus was shown for producing a video disc master which utilized a high power laser in conjunction with a glass disc having a thin film of a relatively low melting point material, such as bismuth, coated thereon. The laser beam was intensity modulated with the video information, and at the point of impingement upon the bismuth film surface, the laser beam, at relatively greater intensities, contained sufficient energy to melt the bismuth film.
The preferred physical properties of a low melting point material, such as bismuth, are that the surface tension of the melted material should cause the molten material to immediately coalesce into small, submicroscopic nodules, thereby leaving an area substantially free of the opaque metal coating. According to the Winslow application, typical "holes" representing video information were on the order of 1 micron.
The single master thus produced cannot, in and of itself, under prior art methods and techniques, be easily utilized to produce hundreds of thousands of replicas rapidly and at low cost. Accordingly, it has been deemed desirable to somehow modify the recorded master so that replication can be easily accomplished.
In the prior art, it has been known to create "masks" with a predetermined pattern which, in turn, could be used in connection with photoetching techniques to create a plurality of duplicate surfaces, each with a similar predetermined pattern in the surface. For example, such a mask could be utilized to selectively expose discs having a very thin metal surface coating to a laser beam, and a plurality of discs having a similar "hole" pattern in the surface could be produced.
Alternatively, a photoengraving process may be employed utilizing a master mask which, through chemical etching techniques, can result in a patterned disc.
Such techniques would not be directly applicable to the needs of the video disc system as presently envisaged because of the costs involved and the time required to create the duplicates or replicas. Since the pattern dimensions closely approximate the wavelength of visible radiation, normal, high-speed photographic duplication techniques would be seriously affected by diffraction effects.
One of the principal advantages in the use of a video disc for recording and reproduction of audio-visual material through a conventional television set over magnetic tape is the opportunity for rapid, mass production of duplicate discs. The relative ease of producing duplicate audio discs versus the methods of producing audio tapes quickly illustrates that discs can be stamped at relatively high speeds in large numbers of presses while each tape record requires the serial recordation on a length of tape of the information to be recorded.
Consequently the relative costs that must be considered as between video tape and video disc are the cost of the material itself, the cost of equipment for transferring the information and the time required for the information transfer. In the audio field, the cost factors clearly favor the disc approach and it has been determined that similar consideration would obtain when providing recorded video information.
In a prior copending application assigned to the assignee of the present invention, Gregg, supra, Ser. No. 735,007, filed Jan. 27, 1969, now abandoned and continued as Ser. No. 571,259, filed Apr. 24, 1975 apparatus was disclosed for embossing information on a sheet vinyl disc. The creation of the stamper was assumed without any detail as to how the stamper might be created. In the video disc system which has been disclosed in the several related patent applications and patents listed, supra, an original master is created through the use of a high-powered laser and a flat disc having a thin film of reasonably low melting point, high surface tension, opaque material. The resulting master, however, has information represented as transparent areas in an opaque surface with the transparent areas arranged in an interrupted generally circular path which, in a preferred embodiment, is arranged in a concentric spiral.
Since the thickness of the heat sensitive opaque layer may be as little as several hundred Angstrom units, the master thus produced cannot be directly used in the stamping, compression molding, injection molding or embossing of a duplicate or replica record. In the above-identified patent applications to Jarsen, and Avanzado, et al., methods were disclosed which resulted in the production of a substantially three-dimensional matrix, wherein the relatively opaque and clear areas are converted into surface discontinuities or bumps which can be preferably 0.7 micron in height. These bumps are of a photoresist material overlying the original master plate. It would be desirable to utilize this photoresist matrix in the creation of a stamper which can be used to emboss, mold or stamp replica discs at relatively low cost in mass production quantities.